Apparatus for determining the density of materials and particularly fluids in containers are known in the prior art. Many such schemes are mechanical and require the measurement of the position or the force on a buoyant float. Other types require the determination of the resonant frequency of a piezoelectric material or the determination of the refracted index of the material. Another type which has the advantage of requiring no moving parts and low cost is a radiation type. One useful form of such apparatus is a radiation type densitometer in which a source of radiation is placed on one side of the material to be measured and a radiation detector placed on the other side. The amount of attenuation of radiation by the material is a measure of the density. Patents such as D. G. C. Hare, U.S. Pat. No. 2,316,239 issued Apr. 13, 1943, P. E. Ohmart U.S. Pat. No. 2,763,790 issued Sept. 18, 1956, M. Lintz et al U.S. Pat. No. 2,898,466 issued Aug. 4, 1959, T. P. Pepper et al U.S. Pat. No. 2,968,729 issued Jan. 17, 1961, W. H. Faulkner et al U.S. Pat. No. 2,922,888 issued Jan. 26, 1960 and D. E. Wright U.S. Pat. No. 3,196,271 issued July 20, 1965 all show one or two detector systems for measuring density. Radiation density measuring devices have encountered difficulties in the past due to several problems. One detector systems have difficulty when the radiation source decays or residue builds up on the apparatus since the detector cannot distinguish between reduction of radiation from a decaying source and increased attenuation from the material being measured. Efforts to solve this problem have resulted in two detector systems wherein a first radiation detector and radiation source are situated with respect to the unknown material and a second radiation detector and source are situated with respect to a reference material and the outputs of the two detectors are subtracted so that the decay of radiation source is eliminated from the equations. The difficulty with these type systems has been that the detector efficiencies usually vary in a way which is not compensated for by a subtracting type system. Furthermore, using a reference material or source requires additional apparatus which may be difficult to accommodate in certain situations such as in aircraft use. Some prior art systems have utilized detectors mounted on the outsides of containers in which the material to be measured is located and such systems have difficulty because of the absorption characteristics of the container itself which must be compensated for. Furthermore, use of a "standard" reference material and subtracting detector signals to compensate for errors in source decay does not compensate for the type of errors which are common to both detector measurement chains.
Radiation detector systems have also encountered difficulty with public acceptance since radiation sources have often been unsafe because of high radiation levels and extra apparatus is needed to provide shielding.